Slide show: Common types of congenital heart defects
Image of heart with atrial septal defect
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Atrial septal defect is a hole that occurs when the muscular wall (septum) separating the top two chambers of the heart (right and left atria) doesn't close properly. This allows oxygen-rich blood from the left atrium to flow into the right atrium, where it mixes with blood that doesn't have adequate oxygen in it. Blood from the left atrium should normally flow into the left ventricle and on to the aorta and the rest of the body.

Minor cases may cause no symptoms and may not require treatment. Larger defects may require surgery or cardiac catheterization to repair the hole.

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